The goal of this proposal is to study the dynamics (the removal, the insertion, and the mobility) of the extracellular matrix acetylcholinesterase (ACHE) in the synaptic cleft in the neuromuscular junction (NMJ) of living mice. In particular, we will study what factors that might regulate its dynamic behavior. We will focus on the role of synaptic activity, a key molecule in the extracellular basal lamina (laminin alpha 4) and in postsynaptic proteins, and the dystrophin glycoprotein complex. Our second goal is to investigate whether the metabolic stability of acetylcholine receptors (AChR) in the postsynaptic membrane is influenced by AChE function. Finally we will study changes in synaptic dynamics of both AChE and AChR at a single synapse. To address these questions we will use the NMJ as a model of synapses in vivo. The use of high-resolution confocal microscopy, in vivo fluorescence imaging assay and the photo-unbinding technique that we developed recently will help us to better understand how the densities of AChE and AChR are maintained and regulated in living synapses. By investigating such questions, we should gain a better understanding of how synaptic alterations occur in less accessible central synapses. Additionally many neuromuscular diseases have either primarily or secondarily major impact on the density of AChRs and AChEs at neuromuscular junctions such as myasthenia gravis. By understanding the regulation of these key synaptic molecules, we expect to define new approaches that might be used to develop effective therapies for these devastating diseases.